The present invention generally concerns methods and apparatus for the continuous production of polymers in a carbon dioxide reaction medium.
Increased environmental concerns and regulations over the use of volatile organic compounds (VOCs) since the late 1980s (e.g. Montreal Protocol in 1987 and the Clean Air Act amendments in 1990) have caused considerable effort being put into finding environmentally benign solvents for industrial use (McHugh, M. A. and V. J. Krukonis, Supercritical Fluid Extraction: Principles and Practice. Seconded, ed. H. Brenner. 1994, Boston: Butterworth-Heinemann). DeSimone et al. at the University of North Carolina-Chapel Hill have shown that supercritical carbon dioxide (scCO2) is a viable and promising alternative solvent (Tc=31.8xc2x0 C., Pc=76 bar) to perform free-radical, cationic and step-growth polymerizations using batch reactors (DeSimone, J. M., Z. Guan, and C. S. Elsbernd, Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science, 1992. 257: p. 945-947). This work has been summarized in several recent reviews (Kendall, J. L., et al., Polymerizations in Supercritical Carbon Dioxide. Chem.Rev., 1999. 99(2): p. 543-563; Canelas, D. A. and J. M. DeSimone, Advs. Polym. Sci., 1997. 133: p. 103-140; Shaffer, K. A. and J. M. DeSimone, Chain Polymerizations in Inert Near and Supercritical Fluids. Trends in Polymer Science, 1995. 3(5): p. 146-153). Indeed, CO2 technology is intended to be commercially implemented by 2006 for the manufacture of Teflon(trademark) by DuPont (McCoy, M., DuPont, UNC RandD effort yields results, in Chemical and Engineering News. 1999. p. 10). The reasons for the intense industrial interest are that CO2 is cheap ($100-200/ton), of low toxicity, non-flammable, and environmentally and chemically benign. In comparison to existing technologies for making polymers, CO2 technology has several significant advantages as it will allow for the elimination of: a) expensive polymer drying steps; (b) expensive wastewater treatment and disposal steps where significant amounts of monomer, surfactants and emulsifiers are generated (Baker, R. T. and W. Tumas, Toward Greener Chemistry. Science, 1999. 284: p. 1477-1478); (c) disposal of xe2x80x9cspentxe2x80x9d organic solvents; (d) handling, storage and shipping of toxic organic solvent; and (e) chain transfer to solvent, i.e., a reaction that may limit the achievable molecular weight of the polymer.
As industrial interest in using scCO2 as a polymerization medium has grown, several disadvantages of batch reactors have been recognized, including: (1) large reactors which are costly at the high pressures of scCO2; and (2) difficulty in recycling the CO2 and the unreacted monomer. Accordingly, there is a need for new ways to carry out the continuous polymerization of monomers in carbon dioxide, particularly liquid and supercritical carbon dioxide. Moreover, there is a need to remove polymer from a high pressure reaction system in a more efficient manner than currently available. In particular, it would be desirable to separate polymer from high pressure reaction fluid with minimal reduction in pressure of the reaction fluid. Such separation would allow the fluid to be more effectively recycled to an upstream reactor.
In one aspect, the invention provides a method for continuously separating polymer from a high pressure fluid stream. The method comprises subjecting the high pressure fluid stream comprising polymer particles to a filter, wherein the filter segregates the high pressure fluid stream from the polymer particles; subjecting the polymer particles to a rotating device which transports the polymer particles away from the filter, wherein the polymer particles are exposed to thermal conditions sufficient to melt the polymer particles and form a seal surrounding at least a portion of the rotating device; and separating the molten polymer from the rotating device. Advantageously, the method is carried out such that the separation of polymer from the high pressure fluid stream occurs under steady-state.
In another aspect, the invention provides an apparatus for continuously separating polymer from a high pressure fluid stream. The apparatus comprises a filter for segregating polymer particles from the high pressure fluid stream; an inlet in communication to the filter to introduce the polymer particles and the high pressure fluid stream thereto; a first outlet connected to the filter for withdrawing the high pressure fluid stream therefrom; a rotating device in communication with the filter for withdrawing the polymer particles from the filter; a heater in communication with the rotating device to melt the polymer particles such that a melt seal is formed around at least a portion of the rotating device; and a second outlet in communication with the rotating device for withdrawing molten polymer therefrom. The apparatus is configured such that the separation of polymer from the high pressure fluid stream occurs under steady-state.
The present invention is explained in greater detail by the embodiments in the drawings herein and the specification set forth below.